Generally, a communication optical transceiver module that requires a wavelength separation of light includes a light emitting element for generating transmission signal light, a lens for focusing light emitted from the light emitting element to an optical fiber, a wavelength separating filter for separating wavelengths of the transmission signal light and reception signal light, a light receiving element for receiving the reception signal light output from the optical fiber, and a lens for focusing the light on the light receiving element (see, for example, Patent Literature 1 mentioned below). In such a communication optical transceiver module, upstream signal light emitted from the light emitting element passes through the lens and the wavelength separating filter, and is focused on the optical fiber. Meanwhile, downstream signal light output from the optical fiber is reflected at the wavelength separating filter, passes through the lens, and is focused on the light receiving element.
Generally, the wavelength separating filter used in the communication optical transceiver module described above is manufactured by forming a dielectric multilayer film on a parallel flat dielectric substrate. As the dielectric substrate, for example, BK7 having a refractive index of 1.5 and the like are used.